1. Field of the Invention
This invention relates to the manufacture of multi-layer printed circuit boards. More particularly, this invention relates to methods for laminating the individual printed circuit boards together to produce a multi-layer printed circuit board.
2. Description of the Background Art
Printed circuit boards comprise a sheet or board of electrically resistive material on which is printed a wiring circuit. Various electrical components such as integrated circuits, discrete components and connectors are then soldered to the wiring circuit via holes drilled through the board. Double-sided printed circuit boards comprise a board having a wiring circuit printed on both sides thereof.
Multi-layer printed circuit boards have been developed which comprise individual single- or double-sided printed circuit boards laminated together. The result obtained is a multi-layered board having outer and inner layers of printed circuits electrically insulated from one another. Holes are drilled through the board at planned locations to allow interconnection of one printed circuit with one or more of the printed circuits. Drill smear existing on the edges of the hole of the printed circuit is removed either through a wet etch-back process or by the plasma desmearing process disclosed in my previous patent, U.S. Pat. No. 4,425,210, entitled "Plasma Desmearing Apparatus and Method", the disclosure of which is hereby incorporated by reference herein.
After drill smear is removed, the holes are then plated through to electrically interconnect the individual printed circuits by either electrolytic plating, electroless plating, immersion plating, or plasma plating, as more particularly disclosed in my previous patent, U.S. Pat. No. 4,474,659, entitled "Plated-Through-Hole Apparatus and Method", the disclosure of which is hereby incorporated by reference herein. The multi-layer board is now ready to receive the various electrical components via the holes in the board. After inserting the leads of the components into the holes, the components are soldered to the printed circuits, either individually or by wave soldering techniques.
From the foregoing brief description, it is evident that the manufacturer of multi-layer printed circuit boards (MPCB) involves many exacting processes in order to produce a structurally and electrically sound MPCB which meets with military and civilian specifications. In regard to my present invention, as set forth hereinafter in more detail, one of the many problems associated with the manufacture of MPCBs is the method for laminating the printed circuit boards to meet such specifications. Usually, failure of the MPCB is the result of improper or inadequate lamination of the layers together which results in, in some areas, no secure lamination during the manufacturing process or actual delamination in the field when the board is in use. Consequently, proper lamination is critical to the successful manufacture of MPCBs.
Presently, there exist many variations in the techniques used to successfully laminate MPCBs. One of the most common techniques for producing MPCBs is to begin with non-conductive boards which are coated on both sides with copper or similar conductive material. Each side of the double-sided board is then coated with an etch-resistant/photo-resist material. Using a photographic negative of the desired printed circuit, the photo-resist is exposed and developed to remove the photo-resist in all areas except for the lines which will constitute the printed circuit. The board is then etched to remove the copper coating in all areas except for such lines, thereby producing copper lines defining the printed circuit. Of course, the printed circuits formed on each side of the double-sided board have been registered with respect to one another. Usually, through registration holes are provided to assure that the printed circuits will be registered when the boards are laminated together.
During the actual step of lamination, the boards are stacked on a platen. An epoxy-impregnated, non-conductive woven material, also known as "B" stage, is placed between each adjacent board and then another platen is placed on top of the sandwiched lamination. The woven material is selected having a certain thickness to assure that the printed circuit on one board is electrically isolated from that of the adjacent board. The bottom platen usually includes registration pins which allow the boards to be registered with respect to one another, thereby also registering the printed circuits.
The sandwiched platens/printed circuit boards/epoxy-impregnated woven material, commonly referred to as a "book", is placed into a heated, hydraulic press. The press is then operated to heat and forcibly compress the book to activate and rapidly cure the epoxy contained within the woven material. After curing the epoxy at the desired temperature and pressure, the book is released from the press and, after cooling, the platens are removed revealing the produced MPCB. After drilling the MPCB, the holes in the MPCB are desmeared and then plated-through by one of the processes discussed above. The MPCB is now ready for receiving the electrical components.
In other variations of the lamination method, the book may comprise inner layers composed of single-sided boards, and/or the adhesive layer may comprise epoxy-glass, acrylic, or the products sold under the trademarks "Polyamide", "Kapton", or "Teflon".
One of the most difficult problems associated with the laminating process is the difficulty of properly heating and compressing the layers of boards and epoxy-impregnated woven material together with assurance that the epoxy will be heated to sufficiently flow and make adhesive contact with all areas of the boards as pressure is simultaneously applied. Specifically, underheating and/or underpressurization may result in some areas never being properly bonded. Conversely, overheating and/or overpressurization will, typically, result in too much of the epoxy being forced out from between the boards and/or over-compression of the woven material. This will produce a thinner, non-conductive layer which does not electrically isolate the printed circuit on one board from that of the adjacent board. Consequently, the desired electrical properties of non-conductance between printed circuits cannot be assured. Further, so much epoxy may have been forced out from between the boards that delamination in the field will occur due to the lack of optimal bonding strength.
Therefore, it is an object of this invention to provide a method which overcomes the aforementioned inadequacies of the prior art and provides an improvement which is a significant contribution to the advancement of multi-layer printed circuit board lamination art.
Another object of this invention is to provide a multi-layer printed circuit board lamination method which provides for more precise and consistent lamination of printed circuit boards than that which can be obtained by previously known methods.
Another object of this invention is to provide a multi-layer printed circuit board lamination method which involves the use of a vacuum chamber to assure that the heating and bonding of the epoxy occurs in a vacuum.
Another object of this invention is to provide a multi-layer printed circuit board lamination method in which the lamination is carried out in a vacuum and with no substantial pressure being applied to the printed circuit board to assure proper thickness and bonding of the epoxy during the process.
Another object of this invention is to provide a multi-layer printed circuit board lamination method which can be accomplished with repeatable accuracy to assure production of multi-layer printed circuit boards in production quantities and meeting the highest specifications.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed method in a different manner or modifying the method within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the method and the detailed description of the preferred method in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.